Peripheral Nervous System

Peripheral Nervous System

Figure 35-16 General organization of the autonomic
nervous system

The peripheral nervous system includes
all nervous tissue outside the central
nervous system. It consists of two functional
divisions: sensory or afferent
division, which brings sensory
information to the central nervous
system, and motor or efferent division, which conveys motor commands to muscles and glands. The efferent
division consists of two components:
(1) somatic nervous system, which
innervates skeletal muscle, and (2) autonomic
nervous system, which innervates
smooth muscle, cardiac muscle,
and glands.

Autonomic Nervous System The
autonomic system governs involuntary,
internal functions of the body
that do not ordinarily affect consciousness,
such as movements of the
alimentary canal and heart, contraction
of the smooth muscle of blood
vessels, urinary bladder, iris of the
eye, and others, plus secretions of
various glands.

Autonomic nerves originate in the
brain or spinal cord as do nerves of the
somatic nervous system, but unlike the
latter, autonomic fibers consist of not
one but two motor neurons. They
synapse once after leaving the cord
and before arriving at the effector
organ. These synapses are located outside
the spinal cord in ganglia. Fibers
passing from the cord to the ganglia
are called preganglionic autonomic
fibers; those passing from the ganglia
to the effector organs are called postganglionic
fibers. These relationships
are illustrated in Figure 35-16.

Subdivisions of the autonomic system
are the parasympathetic and sympathetic systems. Most organs in
the body are innervated by both sympathetic
and parasympathetic fibers,
whose actions are antagonistic (Figure
35-17). If one fiber stimulates an
activity, the other inhibits it. However,
neither kind of nerve is exclusively
excitatory or inhibitory. For example,
parasympathetic fibers inhibit heartbeat
but excite peristaltic movements
of the intestine; sympathetic fibers
increase heartbeat but inhibit intestinal
peristaltic movement.

The parasympathetic system consists
of motor neurons, some of which
emerge from the brain stem by certain
cranial nerves and others of which
emerge from the sacral (pelvic) region
of the spinal cord (Figures 35-16 and
35-17). In the sympathetic division
nerve cell bodies of all the preganglionic
fibers are located in the thoracic
and upper lumbar areas of the
spinal cord. Their fibers exit through
the ventral roots of the spinal nerves,
separate from these, and go to sympathetic
ganglia (Figure 35-17), which are
paired and form a chain on each side
of the spinal column.

All preganglionic fibers, whether
sympathetic or parasympathetic, release
acetylcholine at the synapse with postganglionic
cells. However, parasympathetic postganglionic fibers release
acetylcholine at their endings, whereas
sympathetic postganglionic fibers with
few exceptions release norepinephrine
(also called noradrenaline). This difference
is another important characteristic
distinguishing the two parts of the autonomic
nervous system.

Figure 35-17 Autonomic nervous system in humans. Outflow of autonomic nerves from the central nervous system is shown at left. Sympathetic (red) outflow is from the
thoracic and lumbar areas of the spinal cord by way of a chain of sympathetic ganglia. Parasympathetic (blue) outflow is from the cranial and sacral regions
of the central nervous system; parasympathetic ganglia (not shown) are located in or adjacent to the organs innervated. Most organs are innervated by
fibers from both sympathetic and parasympathetic divisions.

As a general rule the parasympathetic
division is associated with nonstressful
activities, such as resting, eating,
digestion, and urination. The
sympathetic division is active under
conditions of physical or emotional
stress. Under such conditions heart rate
increases, blood vessels to the skeletal
muscles dilate, blood vessels in the
viscera constrict, activity of the intestinal
tract decreases, and metabolic rate
increases. The importance of these
responses in emergency reactions
(sometimes called the fright, fight or
flight response) are described in the
next section. It should be
noted, however, that the sympathetic
division is active also during resting
conditions in maintaining normal blood
pressure and body temperature.